This invention relates to an optical recording medium on which the recording and erasing of information can be done by irradiating the recording medium with light, and more particularly to a rewritable phase-change optical medium which permits information to be recorded thereon at a high speed, and which has a high record rewriting cyclability and a high long-term storage stability.
In a conventional rewritable phase-change optical medium, a recording layer consists of an alloy containing tellurium Te as a main component. In order to record information on this recording medium, a focused laser beam pulse is applied to the recording layer in a crystalline state for a short period of time to partially melt the same and form an amorphous recorded mark quenched and solidified by thermal diffusion. Since the reflectivity of this recorded mark is lower than that in the crystalline state, the former recorded mark becomes optically readable as a recorded signal. In order to erase the recorded mark, it is heated to a temperature not higher than the melting point thereof and not lower than the crystallization temperature thereof by irradiating the recorded mark with a laser beam, to crystallize the recorded mark in an amorphous state, whereby the recording layer can be returned to an unrecorded state.
The known tellurium alloys used for this recording layer include three-element alloys, such as Ge.sub.2 Sb.sub.2 Te.sub.5 (N. Yamada et al, Proc. Int. Sym. on Optical Memory, 1987 p61-66). An optical recording medium having a recording layer of a tellurium alloy has the characteristics that it has a high crystallization speed and enables the overwriting of information to be done at a high speed with a circular single beam by only modulating the irradiation power of the light. An optical recording medium using such a tellurium alloy recording layer is usually provided with dielectric layers having thermal resistance and light transparency on both surfaces of the recording layer, and also a reflective layer of metal such as aluminum capable of reflecting light on the dielectric layer positioned on the side of the recording layer which is opposite to the optical beam entering side thereof. The dielectric layers are adapted to protect the recording layer so that the deformation and abrasion of the recording layer do not occur during a recording operation using an optical beam, and the reflective layer to improve the contrast of a reading signal during a reading operation by the optical interference of an incident optical beam and a reflected optical beam and carry out with ease the formation of a recorded mark in an amorphous state by the cooling effect.
In the above-described structure of an optical recording medium, and, especially, in a rapid cooling structure in which a recording layer and a dielectric layer between the recording layer and a reflective layer are formed to a small thickness of around 20 nm, the deterioration of the recording characteristics ascribed to the repetition of rewriting operations rarely occurs, though the deterioration of the same occurs much in a slow cooling structure in which dielectric layers are formed to a larger thickness of around 200 nm. The rapid cooling structure further has the characteristics that a margin of the erasing power is wider [T. Ohta et al, Japanese Journal of Applied Physics, Vol. 28 (1989) Suppl. 28-3 pp 123-128].
However, a rewritable phase-changeable optical recording medium of a rapid cooling structure has a problem that variation of the thickness of a recording layer and a very fine abrasion thereof are liable to occur when a recording, erasing or rewriting operation is repeated, i.e. a problem that the cyclability of this recording medium is low.